Grimsby returned to his native Duluth, Minnesota, where he began his anchoring career in 1954, serving as an announcer for WEBC Radio. Shortly thereafter, he decided to switch to the growing medium of television, working as a correspondent and news director at various television stations around Minnesota and Wisconsin, including WEAU-TVEau Claire, WISC-TVMadison, and WXIX-TV (now WVTV) Milwaukee. He then spent two years (1959–1961) at KMOX (now KMOV) in St. Louis, before becoming the anchor and news director at ABC-owned KGO-TV in San Francisco, in 1961.

In 1968 Grimsby was brought to WABC-TV in New York City by recently hired news director Al Primo, who the station imported to bring his Eyewitness News format to New York. Grimsby's initial co-anchor was former WCBS-TV newsman Tom Dunn but the man who was most closely identified with him was Bill Beutel, who replaced Dunn in 1970 and co-anchored the news with Grimsby until 1986.

A six-time Emmy Winner, Grimsby was fired from WABC in 1986 and, in an incident recounted by several of his colleagues, including Tom Snyder (who reported the incident on The Late Late Show soon after Grimsby's death[1]), ABC further punished Grimsby by buying a building across from WABC's Lincoln Square studios where three bars Grimsby often frequented stood and evicting the bar owners from the building.

Grimsby was hired by WNBC-TV shortly after his WABC departure, where he served as anchor and reporter. In 1990, he relocated to California where he and George Reading of KMST became the first anchor team on KUSI's newly-launched newscast.

For some time while anchoring at WABC, Grimsby also moonlighted as a reporter for the ABC Radio Network. During this time, Grimsby would serve as a newsreader for the network's five-minute national news updates in the 3:00 PM hour every afternoon, choosing which stories he would report and writing the reports himself. Grimsby, in a 1982 interview with then-colleague John Slattery, called it "one of the best kept secrets in broadcasting." (Outtakes from Slattery's report later aired as part of Tom Snyder's tribute to Grimsby following his death.)

Grimsby was known for beginning his broadcasts with the phrase "Good evening, I'm Roger Grimsby; here[a] now the news," and ending them with the phrase "Hoping your news is good news, I'm Roger Grimsby." Chevy Chase later parodied the opening line on Saturday Night Live's Weekend Update segment with his catchphrase, "Good evening, I'm Chevy Chase and you're not."

Grimsby could be rather funny, himself. His humor was off-the-cuff, often with the typical journalist's sarcastic tone. One famous remark, widely circulated on an industry outtake reel, came after a studio wide-shot caught colleague Mara Wolynski using an extended middle finger as she finished an argument with someone off-screen before her story was introduced on the 5 o'clock edition. At the end of the 6 o'clock edition, Grimsby, with a straight face, looked into the camera and quipped, "Well . . . as Mara Wolynski would say -- 'We're number one.'"[2] He frequently punctuated his banter with other Eyewitness News team members with his humor. One evening he read a story about the proliferation of dog feces around Manhattan, referring to it as a "veritable minefield", then transferred the broadcast to sportscaster Howard Cosell who questioned with "a veritable minefield, Roger", to which Grimsby replied (without skipping a beat): "Don't step in it Howard." Or the time his co-anchor, Bill Beutel, reported a story about a lawsuit in California alleging that a girl had become a nymphomaniac after falling from a San Francisco streetcar. Grimsby chimed in with "Imagine if she had fallen from a train!" As part of the same 1982 piece featured after he died, when asked about embarrassing moments on air, Grimsby said once that he had mispronounced "count".

Grimsby was not immune to foul ups himself though. In one example from the early 1980s, which has since made the rounds on YouTube, Grimsby had fallen victim to a fit of laughter just before a newscast began and it spread to everyone in the studio as the program opened. Grimsby, still trying to compose himself and failing, opened the newscast with, "We are live...hoping your news is good news, I wanna quit now!". He could not get through the lead story, concerning a tuberculosis outbreak, without laughing again and the crew continued to laugh with him. After nearly a minute Grimsby collected himself, took a deep breath, said "okay, composure," and went on with the newscast as if nothing had happened.[3]

His on-air feuds with fellow "Eyewitness News" team members, including Howard Cosell, Geraldo Rivera and gossip columnistRona Barrett (whom he openly called "Rona Rooter") were legendary. He once segued into a Barrett report after a story about garbage by saying "Speaking of Garbage."